When an electric light bulb is turned on, a sudden incoming surge of electrical current encounters the relatively low resistance (e.g. 3 ohms) typically exhibited by the cold bulb filament. The resistance of the filament may be only 1/41 that of its resistance at operating temperature (e.g. 125 ohms), and in such a case the initial current surge is forty-one times that of the operating current. Consequently, a weak or worn filament will often burn out at the moment the bulb is turned on. To remedy this problem and extend light bulb life, various lamp socket insert devices have been provided. These devices typically employ a variable-resistance thermistor, having contact means on either side thereof, which is interposed between the light socket and bulb contacts. When it is cold (e.g. when the electricity is turned off), the insert device exhibits a resistance which is typically higher than the normal operating resistance of the bulb filament. Accordingly, when the light bulb is turned on the initial current surge through the filament is reduced. As the insert device is heated by the current, its resistance drops so that an increased operating current is delivered to the filament. Simultaneously, the filament has typically attained an operating resistance which enables it to withstand the increased operating current.
Several undesirable features are often exhibited by such insert devices. For example, insertion of the device into the socket is often awkward and typically requires the use of either fingers or tools, such as pliers. At times the installer may neglect to electrically disconnect the lamp, and therefore insertion presents a grave danger of electrical shock.
Present light socket insert devices are often relatively thick. Consequently, they conduct heat relatively slowly and exhibit a smaller than desired drop between high and low temperatures. The initial protective resistance may be lower than desired, and thus premature filament burnout may still occur. Or, the operating resistance of the insert may be higher than desired, thereby unduly restricting current to the bulb filament. Power is thus wasted and the insert device is heated to such a high temperature that it may be suitable for use only in porcelain sockets, which are rated for high wattage bulbs, and not in ordinary metallic light bulb sockets. High operating temperature also poses the hazard of arcing. The mechanical stress and high operating temperatures often experienced by present light socket inserts may lead to cracking and reduced insert life.
The relatively large thickness exhibited by certain of the present insert devices may also force the metal base of the light bulb to extend above the top edge of the light socket. Again, a hazard of shock is presented. Insulating washers and other devices have been provided for wrapping about the exposed base in order to reduce this danger. However, use of such insulating devices may be inconvenient and is often neglected.